Upon commercialization of a Blu-ray Disc making use of a blue-ray laser diode, and a high-NA objective lens, an optical disk has nearly reached its limit in terms of resolving power. As a method for concurrently realizing both a further increase in recording capacity, and a further increase in transfer rate, multi-level recording is a favorable candidate. Technologies concerning the multi-level recording are described in, for example, Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6, Patent Document 7, and so forth, respectively.
With Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5, respectively, multi-level level recording is enabled by providing a recording medium wherein reflectance of a recoding mark undergoes successive changes for power of a recording beam irradiated to the recording medium. With Patent Document 6, there is provided a method whereby a position as well as a length of a recording mark is modulated, thereby carrying out multi-level recording.
Meanwhile, with Patent Document 7, it is intended to realize an increase in recording capacity by executing multi-level recording of the phase of an information beam in a holographic memory for storing page data. The outlines of this method are described with reference to FIG. 25. First, at the time of data recording, a laser beam outgoing from a light source 2501 is modulated by a spatial light modulator 2502 before falling on a recording medium 2503, thereby recording information. Herein, the spatial light modulator 2502 is made up of a multitude of pixels, as shown in FIG. 26A, and the respective pixels apply optical phase modulation to the laser beam passing through the central part thereof to be designated as an information beam (in the figure, a white part expresses phase 0, a gray part optical phase modulation according to color strength, and a dark part transmittance 0). No modulation, or suitable optical phase modulation is applied to a portion of the laser beam, passing through the peripheral part thereof, and the portion of the laser beam is designated as a reference beam. At this point in time, an interference pattern between the information beam and the reference beam is recorded in the recording medium 2503. Then, at the time of regeneration, the spatial light modulator 2502 does not execute modulation to a portion of the laser beam, passing through the central part thereof, and the portion of the laser beam is designated as a DC beam, as shown in FIG. 26B, applying the same modulation as applied at the time of recording to the portion of laser beam passing through the peripheral part thereof, whereupon the laser beams passing through the respective parts are irradiated to the recording medium. By so doing, the information beam is generated by irradiation with the reference beam, and a beam in such a state as the DC beam with the information beam superimposed thereon is detected by an image sensor 2504. The beam detected at this point in time is due to interference occurring between the information beam, and the DC beam, and the respective pixels in the information beam are subjected to intensity modulation corresponding to the respective phases thereof before being detected. Further, the phase of the DC beam is uniformly changed by the spatial light modulator 2502, and the same measurement is executed, whereupon a phase relationship between the information beam, and the DC beam, in the respective pixels, will undergo a change, so that beams differing in intensity pattern from each other are detected by the image sensor. Thus, by obtaining outputs of the image sensor 4 when the DC beam has phases in 4-different states, 0, π/2, π, π3/2, it is possible to obtain phase values of the information beam, in the respective pixels. Further, with Patent Document 7, intensity modulation, together with optical phase modulation, is executed, thereby enhancing a multi-level degree.
Further, with Patent Document 9, multi-level recording, and multiplex recording, using a polarization state of light, are realized in a holographic memory for storing page data by use of a medium sensitive to polarization. The outlines of this method are described with reference to FIG. 27. First, the case of the multi-level recording is described. At the time of recording, a laser beam outgoing from a light source is split into a signal beam, and a reference beam by a non-polarizing beam splitter 2701, and the signal beam passes through a spatial light modulator 2502 to be turned into a predetermined linearly polarized light beam on a pixel-by-pixel basis in the modulator before being irradiation to a recording medium. The reference beam in a state of a linearly polarized light beam, without being subjected to modulation, is irradiated to a location identical to a location irradiated with the signal beam, on a recording medium 2503. Then, at the time of regeneration, only the reference beam, in the polarization state identical to that of the reference beam at the time of recording, is irradiated to the recording medium 2503. Thence, a light beam having the polarization state identical to that of the signal beam at the time of recording is emitted from the recording medium 2503. The light beam is split into portions by a polarization beam splitter 2702, and the respective portions are detected by CCD cameras 2703, 2704, respectively. Because the CCD cameras 2703, 2704 each output values according to respective magnitudes of a p-regeneration beam and an s-polarization component, contained in the polarization state, on a pixel-by-pixel basis, the polarization state is estimated from a ratio of the magnitude between the respective components, on the pixel-by-pixel basis. Accordingly, if the state of the linearly polarized light beam is modulated in multi-stages, outputs in multi-stages, according thereto, can be obtained on the pixel-by-pixel basis, so that recording•regeneration of multi-level information is enabled. Next, the case of multiplex recording is described hereinafter. In the case of multiplex recording, the spatial light modulator 2502 shown in FIG. 27 executes the same intensity modulation as in the past, and the signal beam, and the reference beam are irradiated to the recording medium 2503 with the respective polarization states of the signal beam, and the reference beam being kept constant within a beam plane (for example, the signal beam in the p-polarization state, and the reference beam in the s-polarization state), thereby executing hologram recording. Subsequently, the p-polarization of the signal beam is changed through 90° (that is, changed to the s-polarization) by use of a half-wave plate (not shown), and so forth, and another data modulation is executed by the spatial light modulator, thereby executing hologram recording at an identical location again. At the time of regeneration, irradiation is made with only the reference beam in a polarization state identical to that of the reference beam at the time of recording. By so doing, a light beam of the signal beam in the state of the p-polarization with the signal beam in the state of the s-polarization superimposed thereon is emitted from the recording medium. Accordingly, the light beam is split into the p-polarization, and the s-polarization by the polarization beam splitter 2702, and data blocks recorded in the respective polarization states are detected by the CCD cameras 2703, 2704, respectively. That is, multiplex recording•regeneration can be realized by use of the s-polarization state, and the p-polarization state.
Meanwhile, as another approach to enhancement in recording capacity, a method has been under studies whereby light is condensed close to a diffraction limit on a recording medium, as is the case with an optical disk such as a common CD, and DVD, and two light beams opposing each other are focused at locations identical to each other, thereby recording interference fringes (standing waves) of the two light beams in the vicinity of a focusing point (refer to, for example, Non-patent Document 1, and Patent Document 8). This method is advantageous in that a multilayered recording layer is easily formed while surface recording density is the same in degree as that for a conventional optical disk, an increase in capacity is easily realized since multiplex recording is enabled, severe tolerance as required in the case of hologram recording of page data is not required although it is a method for recording interference, and mounting can be carried out with relative ease.